Natural gas generally is sold by volumetric measurement, and one way to measure the volume of a flowing gas is to pass it through an orifice of a known size to measure the pressure drop across it. An orifice meter comprises a housing within which a disc with an orifice is supported in a gas pipeline, the housing being fitted with pressure taps for measuring pressure immediately upstream and downstream of the orifice. Because the orifice is subject to wear by impacting of sand, lime scale, and other foreign particles in the flowing stream, it must be replaced at frequent intervals to ensure accuracy and measurement. A basic orifice meter may comprise simply a housing in which a carrier holding an orifice disc is received. A junior type orifice meter may include means for moving the carrier and disc to a displaced compartment which is accessable from the outside for servicing. In either case, the pipeline must be shut down or bypassed while the worn orifice disc is removed from the body and replaced with a new disc. In the more sophisticated, or senior type orifice fittings, the orifice disc carrier is moved out of the flow passage to a displaced compartment of the housing, which is then isolated from the flow passage so that flow can continue while the orifice disc is being replaced, with the parties assuming that there is a continuing, constant flow rate based on previous measurements.
Economics may dictate that the purchase of an orifice meter be limited to the less expensive basic or junior type fitting. However, if circumstances later justify the acquisition of a more expensive embodiment, the only generally available choice is to remove the old meter and replace it with the new, with the hope that the old meter has some salvage or resale value to minimize the new investment.